1. Field of the Invention
The present invention relates to an optical communications network, and to a passive optical subscriber network in particular, and relates to a power supply controlling method, a station-side equipment, a subscriber-side equipment, a semiconductor chip of a station-side equipment, and a semiconductor chip of a subscriber-side equipment in the optical communications network.
2. Description of the Related Art
A telecommunications network that connects a building (operator's office) owned by a telecommunications operator and homes of subscribers is called an access network. Due to recent increasing communication capacities, employing optical communications, optical access networks that can transfer vast amounts of information are getting the mainstream.
A passive optical subscriber network (PON: Passive Optical Network) is one form of an optical access network. A PON consists of one station-side equipment that is installed within an operator's office, several subscriber-side equipments that are respectively installed within the homes of subscribers, and an optical splitter. The station-side equipment and the respective subscriber-side equipments are connected to the optical splitter with an optical fiber (refer to, for example, “Technical Basics Lesson: GE-PON Technology” in NTT GIJUTU Journal (NTT Technical Journal), August 2005).
The connection between the station-side equipment and the optical splitter is made by using a single-core optical fiber. This single-core optical fiber is shared with multiple subscriber-side equipments. Further, the optical splitter is an inexpensive passive equipment. In this way, a PON is extremely cost effective, and further, maintenance is also simple. Therefore, the introduction of PONs is proceeding rapidly.
Various types of multiplexing technologies are used in PONs. Typical multiplexing technologies that are used in PONs are Time Division Multiplexing (TDM) that allocates short slots on the time axis to each subscriber, Wavelength Division Multiplexing (WDM) that allocates different wavelengths to each subscriber, and Code Division Multiplexing (CDM) that allocates different codes to each subscribers, and the like. A TDM-PON, that uses TDM among these multiplexing technologies, is currently most widely used. The TDM-PON will be described with reference to FIG. 3. FIG. 3 is a schematic diagram for explaining for explaining a conventional example of a TDM-PON.
A PON 11 that is an optical communications network has a station-side equipment (OLT: optical line terminal) 31 and subscriber-side equipments (ONUs: optical network units) 61-1 through 61-3. The OLT 31 and the respective subscriber-side equipments 61-1 through 61-3 are connected with an optical fiber via an optical splitter 50.
Communication from the respective ONUs 61 toward the OLT 31 is called upstream communication. Further, communication from the OLT 31 toward the respective ONUs 61 is called downstream communication. Here, mainly downstream communication will be described, and detailed explanation of upstream communication and illustration of functional blocks needed for upstream communication are omitted.
The OLT 31 consists of an interface converting section (I/F) 32, a transmission buffer section 34, a transmitting section 36, and a station-side control section 41.
The interface converting section 32 carries out communications protocol processing with an upper network 20 serving as an external network. The interface converting section 32 converts downstream data signals, that are sent-in from the upper network 20, into downstream data signals of the format defined by PON protocol, and sends the converted data signals to the following transmission buffer section 34. The downstream data signals are signals to be transmitted from the upper network 20 to user equipments 90-1 through 90-3, i.e., are signals to be transmitted by downstream communications, and include information requested by users.
The transmission buffer section 34 accumulates, for a predetermined time period, the downstream data signals that have been sent from the interface converting section 32. After the predetermined time period elapses, the accumulated downstream data signals are sent to the transmitting section 36.
The station-side control section 41 generates downstream control signals, and sends them to the transmitting section 36. The control signals are signals that the OLT 31 and each ONU 61 exchange. The OLT 31 can send commands to each ONU 61 by transmitting downstream control signals to each ONU 61. Further, by receiving upstream control signals from each ONU 61, the OLT 31 can know of the statuses of the respective ONUs 61. In order to carry out stable communications, these control signals need to be transmitted and received periodically between the OLT 31 and the respective ONUs 61.
The transmitting section 36 assigns, to the downstream data signals and the downstream control signals, identifiers that indicate the destination ONU 61. These identifiers are allocated individually to the respective ONUs 61. By using the identifier, each ONU 61 can identify whether or not a received downstream signal (a downstream data signal or a downstream control signal) is addressed to itself.
After assigning identifiers to the downstream signals, the transmitting section 36 converts the downstream signals into downstream optical signals. The transmitting section 36 transmits the downstream optical signals toward the respective ONUs 61.
The ONU 61 consists of a receiving section 62, an interface converting section (IN) 64, an electricity supply section 66 and a subscriber-side control section 81.
The receiving section 62 has a photoelectric converting function, a signal extracting function, and a signal sorting function. The photoelectric converting function is the function of converting the downstream optical signals that are received from the OLT 31 into downstream electric signals. The signal extracting function is the function of reading the identifiers included in the downstream electric signals, extracting the signals that are addressed to itself, and discarding signals of other addresses. Further, the signal sorting function is the function of sorting the signals addressed to itself into downstream control signals and downstream data signals, and sending the downstream control signals to the subscriber-side control section 81, and sending the downstream data signals to the interface converting section 64.
The interface converting section 64 converts the downstream data signals into data signals of the format that is defined by the communications protocol with the user equipment 90. The converted downstream data signals are sent to the user equipment 90.
The electricity supply section 66 supplies electric power to the receiving section 62.
The subscriber-side control section 81 executes the commands described in the downstream control signals. For example, the transmission timing of an upstream signal is described in a downstream control signal. The subscriber-side control section 81 transmits the upstream signal in accordance with the transmission timing, and prevents collision of upstream signals from multiple ONUs.
The transmission of downstream signals is described next. The numbers given to the downstream signals in FIG. 3 express the identifiers that indicate the destination ONU. “1” indicates that the destination ONU is the first ONU 61-1. “2” indicates that the destination ONU is the second ONU 61-2. “3” indicates that the destination ONU is the third ONU 61-3.
The downstream optical signals (shown by arrow S131 in FIG. 3) that are time-division-multiplexed at the OLT 31 are branched at the optical splitter 50. The downstream optical signals that have been branched at the optical splitter 50 (shown by arrows S150-1 through S150-3 in FIG. 3) include the same contents before and after passing through the optical splitter 50. However, the strength of the downstream optical signal is reduced in accordance with the number of times the signals are branched at the optical splitter 50. The branched downstream signals are received at the respective ONUs 61. At each ONU 61, the identifier described in the downstream optical signal is compared with the identifier allocated to itself As a result of the comparison, signals that match are sent to the following user equipments 90, whereas signals that do not match are discarded.
However, in the above-described conventional PON, each ONU needs to receive all of the downstream optical signals that are sent-in from the OLT, whether or not the signals are addressed to itself. Each ONU carries out reading of the identifiers described in the downstream optical signals, identifying whether or not the signals are addressed to itself, and discarding signals that are addressed to others. Accordingly, even when there are no downstream optical signals addressed to itself at all, the ONU needs to carry out the receiving operation at all times, and as a result, electric power is consumed wastefully.
The downstream optical signals arrive at the ONU in greatly attenuated state by the branching at the optical splitter and the transmission over the optical fiber. Therefore, the receiving section of the ONU is composed of a circuit that consumes much power and amplifies the downstream signal whose strength is low. Accordingly, the proportion of the power consumption of the receiving section is larger compared to the power consumption of the entire ONU.